It sounds as if the sand is getting too dry, Sc. I keep plants in unheated plunge beds filled with sand and I notice that if it dries out I get a hard skin forming on the surface. In your case, is 50mm of sand over the cable enough? It is not very deep.

Thanks Bibaholic. I had thought of that but 50mm is the recommended depth. Parasene enclose instructions and stress to use horticultural sand but I have a feeling that there must be a better, lighter and more absorbent alternative which won't go solid. I was wondering whether peat or potting compost would warm up the same. What do you think?

Not sure about using a humus rich medium, Superscouse, for one thing iit could encourage the growth of too many weeds. I always use an inert material. As I said, a sand plunge (building center sharp sand) and I also use grit trays with 6mm grit. I have no experience of heated plunges but would have thought sand the best. Did you have a thermometer actually in the sand? When I did engineering at school (distant memories) and we did metal castings the sand always came out hard and solid.

You mat well be right Bulbaholic, I hadn't considered the weeds! I think the problem with ordinary sharp sand is the lime content which is why it goes hard. However, if it stays damp it should stay loose like in the bag. However, as soon as it dries, it goes hard and the cable is at risk of overheating. In the greenhouse, it is difficult to keep it evenly moist.